Test runs took place in 1984 and full service began in 1985. When the line opened, 12 sets operated individually as two-car units. In 1986, two more sets were added, allowing sets to be coupled to form four-car units as ridership grew. All trains operate on a automatically without human intervention. Although they are capable of unmanned operations, as in Vancouver and Detroit, the TTC opted to use one-man operation on all trains, a practice which was later used on heavy-rail Toronto Rocket sets on Line 4 beginning in 2016. Trains on Line 1, Line 2, on the other hand, carry two personnel on board (an operator and a guard) while in service.[2]

Since the retirement of the remaining H-series trains in 2014, the S-series trains have been the oldest in operation on the entire subway system. They are also the only TTC rapid transit trains with a painted livery since the G-series, consisting of a unique lowercase "rt" logo, referring to the line's original name of "Scarborough RT". Since 2015, the cars have been undergoing refurbishment and repainting to prolong their lifespan until at least 2023 while the Scarborough extension to Line 2 is under planning. The new exterior of each car features a vinyl wrap with a blue livery (the line's colour on route maps), the Line 3 symbol, and the names of the line's six stations written on the top edge on both sides, in an effort to promote the line as a whole.